Hylo LLVM builds

GitHub CI based builds of LLVM libraries, compatible with the Swift runtime.

These builds contain a set of libraries suitable for compiler development, and a very minimal set of tools such as llvm-config and lld. If you're looking for a compiler binary (e.g. clang), look elsewhere!

Both the “Debug” and “MinSizeRel” builds have assertions enabled; without assertions, during development it's too easy to violate LLVM's preconditions (some of which are undocumented or incorrectly documented)

Swift compatibility is mostly irrelevant except when it comes to Windows, where these builds of LLVM always link against a release-mode multithreaded DLL C runtime and have iterator debugging disabled (_ITERATOR_DEBUG_LEVEL=0) for C++.